1. Technical Field
The present invention relates to a method for manufacturing contact lenses; more particularly, the present invention relates to a method for manufacturing contact lenses having excellent oxygen permeability and water content without using solvent.
2. Description of Related Art
Contact lenses can be generally classified into two categories, soft contact lenses and hard contact lenses, and the soft contact lenses can be further classified into hydrogel contact lenses and silicon hydrogel contact lenses.
The hydrogel contact lenses are made from hydrogel materials, such as poly-2-hydroxyethyl methacrylate (p-HEMA). The strength of 2-hydroxyethyl methacrylate can be enhanced by adding crosslinking agent like ethylene glycol dimethacrylate (EGDMA). Since the water content of the p-HEMA is only about 38.8%, one or more than one kinds of hydrophilic monomers, for example N-vinylpyrrolidone, N,N-dimethylacrylamide and methyl acrylic acid, are added to enhance the water content of such materials. With the addition of the hydrophilic monomer, the water content of the contact lenses can increase from 38% up to 80%. However, the higher water content of the contact lenses is, the lower tension and toughness thereof become. Thus, the water content of the contact lenses is generally controlled in the ranges from about 45% to 58 On the other hand, the real oxygen permeability (DK) of the contact lenses can only reach about 15-35 regardless any improvement of the water content thereof.
When wearing contact lenses with higher oxygen permeability, wearers feel more comfortable because sufficient oxygen will be permeated through the contact lenses from the environment air to reach the cornea of the wearers. In general, there are three methods for increasing oxygen permeability (DK) of contact lenses: (1) thinning the thickness of contact lenses; (2) using material with high water content; and (3) using material with high oxygen permeability. However, the above-mentioned methods (1) and (2) will decrease mechanical strength of the contact lenses.
In addition, the synthesis method of silicon hydrogel and hydrogel can be processed by thermal-reaction and UV-reaction depending on the initiators used respectively. Generally, the reaction time of thermal-reactive silicon hydrogel is about 2-3 hours and the reaction temperature is about 100-120° C. It took time to mold the contact lenses, and mass evaporation of the solvent during the molding process resulted in unexpected surface texture on the contact lenses.
Therefore, the present invention is to provide a method for manufacturing contact lenses to eliminate the aforesaid disadvantages of the prior art.